1. Field of the Invention
The present invention relates to a lens assembly equipped with an optical member and a ceramic lens frame having a hollow part into which the optical member is inserted, and to a method of disposing the optical member in the ceramic lens frame of the lens assembly.
2. Description of the Related Art
In recent years, automobiles have been provided with cameras. Meanwhile, nowadays, automobiles are equipped with display screens thanks to the widespread use of car navigation systems. Therefore, many of cameras installed in automobiles can display the state of a blind spot for a driver on a display screen provided in front of the driver's seat. Incidentally, automobiles may be left outdoors in hot summer and in freezing winter and therefore, a camera installed in an automobile is required to operate properly over an extremely wide range of temperatures. In order to guarantee the proper operation of a vehicle-mounted camera, a lens assembly employed in such a camera needs to be not only lightweight and robust as generally required, but also resistant to stretching and shrinking with temperature for the purpose of preventing the occurrence of a blur due to a change in temperature. To meet these needs, ceramic lens frames have been devised (see, for example, Japanese Patent Application Publications No. 2006-284991 and No. 2006-292927). Also, the inventors of the present application have proposed to apply porous ceramic to an optical component (see, for example, Japanese Patent Application Publication No. 2007-238430). The porosity of the porous body described in the specification of Japanese Patent Application Publication No. 2007-238430 is equal to or more than 10%.
Further, for instance, Japanese Patent Application Publication No. 2007-279557 describes a lens assembly having such a structure that lenses and spacing rings are inserted into a lens barrel through the front opening of the lens barrel, and a pressing ring is screwed on a front part of the lens barrel, thereby fixing the lenses to the lens barrel. A structure in which a ceramic lens frame is applied to the structure of Japanese Patent Application Publication No. 2007-279557 will be described.
FIG. 1 is a diagram that illustrates an example of the structure of a lens assembly having a lens frame made of ceramic including porous ceramic. The structure of the lens assembly will be briefly described with reference to FIG. 1.
A lens assembly 1 illustrated in FIG. 1 is provided with a lens frame 10 made of ceramic, and the lens frame 10 has a hollow part 100 having an object-side opening 101 and an image-forming-side opening 102. Formed on an object-side periphery of the lens frame 10 is a male thread SR1. From the object-side opening 101, lenses L1 trough L4 and spacing rings SP1 through SP3 are inserted while being aligned along the optical axis. In this example, the lenses L1 trough L4 and the spacing rings SP1 through SP3 are alternately disposed and sequentially inserted into the hollow part 100 of the lens frame 10.
Furthermore, the lens assembly 1 illustrated in FIG. 1 is provided with a pressing ring 11 that fixes the lenses L1 trough L4 and the spacing rings SP1 through SP inserted into the hollow part 100 of the lens frame 10, by pressing them from the side where the object-side opening 101 is formed. The pressing ring 11 has: a mounting opening 110 into which an object-side part of the lens frame 10 is inserted; and an optical opening 111 for making a central part of the lens L1 exposed. Among the lenses inserted into the lens frame, the lens L1 is the one disposed at the position closest to the object. Formed on an inner wall on a mounting opening side of the pressing ring 11 is a female thread SR2 in which the male thread SR1 is engaged. When the male thread SR1 is engaged in the female thread SR2, the edge of an object-side surface of the lens L1 disposed at the position closest to the object on the object side is pressed by the pressing ring 11.
The lens assembly 1 shown in FIG. 1 is assembled by using the pressing ring 11 that presses the lenses L1 trough L4 and the spacing rings SP1 through SP3 in the lens frame 10 toward the image-forming-side opening.
When the material of the lens assembly 1 illustrated in FIG. 1 is ceramic, the lens assembly 1 has such a drawback that it is more difficult to smoothly insert the lenses L1 trough L4 and the spacing rings SP1 through SP3 as compared to when the material is a metal or the like, because the inner peripheral surface of the lens frame 10 made of ceramic is uneven. If optical members such as the lenses and spacing rings cannot be smoothly inserted into the hollow part 100 of the lens frame 10, the rim of any of the optical members may be chipped or the optical members may not be disposed at predetermined positions within the lens frame 10. When the lens assembly has the optical members whose rim is chipped or which are not placed at correct positions, it is impossible for a camera mounted with such a lens assembly to deliver appropriate optical performance.